The present invention is a method for the preparation of hydrophilic silica gels with high pore volume. In the method a hydrophilic silica hydrogel is treated with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the silica hydrogel. Then, the hydrophobic silica gel is heated in the presence of an oxidizing atmosphere to a temperature sufficient to reduce the hydrophobicity imparted by the surface treatment thereby producing a more hydrophilic silica gel having high pore volume.
Silica gels are typically prepared by wet processes where, for example, a sodium silicate is gelled either under acidic or neutral conditions resulting in a three-dimensional network which partially collapses upon drying unless special precautions are taken to reduce or prevent such collapse. By definition gels from which the liquid medium has been removed with the structure being compressed and the porosity reduced at least to some degree by the surface tension forces as the liquid is removed are referred to as xerogels. Typically such xerogels have surface areas within the range of 50 to 600 m.sup.2 /g and pore volumes within a range of about 0.5 to 1 ml/g. In order to obtain silica gels having higher porosity, typically referred to as aerogels, it is generally necessary to convert the hydrogel to an organogel such as an alcogel and then heat the organogel in an autoclave to above the critical point of the liquid so that there is no liquid vapor interface, releasing the vapor, and thereby avoiding collapse or change in structure. Although this method for producing aerogels can produce silica aerogels having surface area and porosity greater than that typically associated with xerogels, it is a costly method and therefore has limited utility.
The present invention provides a convenient method whereby a hydrophilic silica gel having high porosity can be formed without resorting to procedures such as that described above for making aerogels.
Brown, U.S. Pat. No. 3,024,126, teaches a method for making a pre-formed reinforcing silica filler hydrophobic by treating it in an organic solvent with an organosilicon compound, such as an organosilane or low-molecular weight organosiloxane containing 0.1 to 2 total hydroxyl and/or alkoxy radicals per silicon atom, and a small amount of amine, quaternary ammonium, or organometallic compound.
Lewis, U.S. Pat. No. 3,979,546, teaches a method for making the surface of reinforcing silica fillers hydrophobic through the use of alpha-alkoxy-omega-siloxanols with alcohols under mild conditions. The fillers taught are pre-formed solids.
Tyler, U.S. Pat. No. 3,015,645, teaches the making of hydrophobic silica powders by reacting an organosilicon compound such as dimethyldichlorosilane or trimethylmethoxysilane with an silica organogel in the presence of an acidic catalyst and then removing the volatile materials. The method requires the preparation of a silica hydrogel which is converted to an organogel by replacing the water in the silica hydrogel with an organic solvent.
Lentz, U.S. Pat. No. 3,122,520, teaches a procedure where an acidic silica hydrosol is first heated to develop a reinforcing silica structure and then mixed with an organosilicon compound, an acid catalyst, and a water-immiscible organic solvent to produce a hydrophobic silica filler.
Burns et al., U.S. Pat. No. 5,708,069, describe a method for preparing hydrophobic silica gels under neutral conditions. The method comprises two steps, where in the first step the pH of a silica hydrosol is adjusted with a base to within a range of about pH 3 to pH 7 to facilitate formation of a silica hydrogel. In the second step the silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the silica hydrogel. In a preferred method, in a third step the hydrophobic silica gel is contacted with a sufficient quantity of a water-immiscible organic solvent to convert the hydrophobic silica hydrogel into a hydrophobic silica organogel.
Burns et al., U.S. Pat. No. 5,789,514, describe a method for preparation of hydrophobic silica gels under acidic conditions. The method comprises two steps, where in the first step a silica hydrosol is contacted with a sufficient concentration of a strong mineral acid at a pH less than about pH 1 to form a silica hydrogel. In the second step the silica hydrogel is contacted with described organosilicon compounds in the presence of a catalytic amount of a strong acid to effect hydrophobing of the silica hydrogel thereby forming a hydrophobic silica hydrogel. In a preferred method the hydrophobic silica hydrogel is contacted with a sufficient quantity of a water-immiscible organic solvent to convert the hydrophobic silica hydrogel into a hydrophobic silica organogel.
Burns et al., U.S. Pat. No. 5,789,495 describe a method for making organosilicate-modified silica gels at a pH within a range of about pH 3 to pH 7. The method comprises two steps, where in the first step the pH of an organosilicate-modified silica hydrosol is adjusted with a base to within a range of about pH 3 to pH 7 to facilitate formation of an organosilicate modified silica hydrogel. In the second step the organosilicate-modified silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the organosilicate-modified silica hydrogel to form a hydrophobic organosilicate-modified silica hydrogel.
Burns et al., U.S. Pat. No. 5,807,501, describe a method for making neutral-aged hydrophobic organosilicate-modified silica gels. The method comprises three steps, where in the first step an organosilicate-modified silica hydrosol is contacted with a strong mineral acid at a pH less than about 1 to form an organosilicate-modified silica hydrogel. In the second step the organosilicate-modified silica hydrogel is aged at a pH within a range of about pH 3.5 to pH 8. In the third step the neutral-aged organosilicate-modified silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the organosilicate-modified silica hydrogel to form a hydrophobic organosilicate-modified silica hydrogel.
Jansen et al., U.S. Pat. No. 5,795,556, describe a process for preparation of xerogels involving acidifying an aqueous waterglass solution, polycondensing the resulting silica by addition of a base, removing the water from the gel, reacting the gel with a silylation agent, and drying the gel.
Jansen et al., U.S. Pat. No. 5,647,962, describe a method for preparing xerogels involving acidifying an aqueous waterglass solution, polycondensing the silicic acid produced by adding a base to form a gel, removing the water from the gel by extractive distillation with an organic solvent, reacting the gel with a silylation agent, and drying the gel.
Schwertfeger et al., Chem. Mater. 1995, 7, 1909-1914, describe the pyrolysis of organically substituted aerogels in a standing atmosphere of argon resulting in the formation of carbonaceous structures covering the inner surface of the aerogels.
The present invention is a method for the preparation of hydrophilic silica gels having high pore volume. Such hydrophilic silica gels are useful in any of those applications for which high surface area and high pore volume silica are typically used and particularly, for example, as rheology control agents in silicone and organic fluids and silicone rubber compositions such as sealants.